Global warming is the steady increase in the average temperatures of Earth's atmosphere (and Earth's surface). The measurements are taken over long duration of time. Earth has been getting warmer much faster in the last two decades as compared to the temperature rises in the previous century.
One of the main reasons for the warming is the increased emissions of greenhouse gases, by people and their activities. These gases are water vapor, carbon dioxide, methane, chloroflurocarbon (CFC) and Ozone. It seems even the clouds could contribute to the radiation levels.
Deforestation and fossil fuel burning are some causes that increased the amounts of these gases in the atmosphere. Burning of gas, coal, oil, flares, cement production etc are reasons too.
Global warming has an effect on the climate at many locations, affecting crops and human system. There have been many economic loses.
There are some efforts by various governments for reducing vehicular pollutions. There can be more mitigation activities like carbon sink, natural sink (reforestation), increasing energy efficiency. Utilizing more of nuclear energy and renewable energy sources can also mitigate greenhouse gas amounts.